PCBs are recalcitrant due their chemical stability and hydrophobicity. In PCB-contaminated sediments, biologically-mediated dechlorination results in reduced toxicity and less bioaccumulation. In the environment, however, these compounds biodegrade slowly and incompletely, making intrinsic bioremediation less desirable for the management of PCB-contaminated sites. We have been working on a technology to stimulate the in situ reductive dechlorination of PCBs in contaminated sediment. Presently, the total number of PCB-dechlorinating microbes in natural samples cannot be quantified; therefore, we cannot determine whether enrichment and/or stimulation techniques result in the growth of dechlorinating bacteria in the environment. If these organisms are identified, molecular-based techniques could be developed to determine which organisms are present at a particular site and how their numbers change with time. In addition, without PCB-dechlorinating bacterial isolates or highly enriched and defined dechlorinating microbial cultures, our Understanding of the physiology of PCB-dechlorination (e.g., growth and biodegradation rates, nutritional needs, etc.) is limited, and thus our efforts to engineer a sediment bioremediation system are significantly hampered. In this proposed research we will: (1) develop highly enriched PCB-dechlorinating cultures (2-3 populations) or obtain PCB-dechlorinating isolates from Baltimore Harbor, Hudson River, Palos Verdes, and Waukegon Harbor sediment and identify the dechlorinators present, and (2) investigate the H2 affinity and threshold, electron donor specificity, electron acceptor specificity, and PCB congener specificity of these organisms.